Alkynyl−B(dan)s in Various Palladium-Catalyzed Carbon−Carbon Bond-Forming Reactions Leading to Internal Alkynes, 1,4-Enynes, Ynones, and Multiply Substituted Alkenes

Tomohiro Tani, Yuuki Sawatsugawa, Yusuke Sano, Yo Hirataka, Naomi Takahashi, Sadahiro Hashimoto, Tetsuya Sugiura, Teruhisa Tsutimoto

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Abstract

It was found that the C(sp)−B(dan) bond of alkynyl−B(dan)s can be directly used for palladium-catalyzed carbon−carbon bond-forming reactions with aryl(alkenyl) halides and allylic carbonates as electrophiles, thus delivering unsymmetrical internal alkynes and unconjugated 1,4-enynes, respectively. With acyl chlorides as electrophiles, ynone synthesis is also promoted by a palladium catalyst with the assistance of a copper co-catalyst. These reactions can be achieved as more convenient one-pot reactions, without isolating the alkynyl−B(dan) formed in situ by the zinc-catalyzed dehydrogenative borylation of alkynes with HB(dan). In addition to direct C(sp)−B(dan) bond transformations, the C≡C bond in an alkynyl−B(dan) proved to be a promising scaffold for the construction of a multisubstituted alkene, which is synthesized by diboration of the C≡C−B(dan) moiety, leading to a triborylalkene followed by iterative regio- and stereoselective Suzuki−Miyaura cross-coupling reactions. As one example, the synthesis of the ethene with four different aryl groups, p-MeC 6 H 4 , p-MeOC 6 H 4 , p-NCC 6 H 4 , and p-F 3 CC 6 H 4 , was attained in high overall yield of 64% in six steps starting from the terminal alkyne, p-MeC 6 H 4 C≡CH. Besides these synthetic applications of the alkynyl−B(dan), the scope of the alkynyl substrate in the zinc-catalyzed dehydrogenative borylation was expanded to enhance the reliability as a provider of the alkynyl−B(dan). Consequently, 42 alkynes were found to participate in the dehydrogenative borylation as substrates; these are alkyl-, alkenyl-, aryl-, heteroaryl-, ferrocenyl-, silyl-, and borylalkynes, with or without a variety of functional groups. Lastly, a new method for preparing HB(dan), as a sulfide-free, cost-saving, and reaction-time-saving route, is disclosed. (Figure presented.).

Original languageEnglish
Pages (from-to)1815-1834
Number of pages20
JournalAdvanced Synthesis and Catalysis
Volume361
Issue number8
DOIs
Publication statusPublished - 16 Apr 2019

Keywords

  • Alkynes
  • Allylic compounds
  • Boron
  • C−C coupling
  • Homogeneous catalysis

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